Aquatic Plants for Acid Mine Drainage Remediation in Simulated Wetland Systems

Aquatic plant is an important component of a constructed wetland system for treating acid mine drainage (AMD). This study was conducted to investigate the remediation effects of planting three aquatic plants species on AMD quality in simulated wetland systems. Simulated wetland systems were constructed using 10-L plastic containers as growth media comprising mixed-organic substrates and aquatic plant species as planting treatments. The treatments involved individual plantings with Fimbristilys hispidula (Vahl) Konth, Mariscus compactus (Retz) Druce, and Typha angustifolia L., and mixed-planting with a combined three-plant species. As the control was the unplanted media. The plants were continuously flooded with very acidic AMD collected from a mine pit in PT Tambang Batubara Bukit Asam, South Sumatra. During the experiment, the acidity (pH), oxidation reduction potential (Eh), and electrical conductivity (EC) of the flooding AMD were measured after 24 hours of the flooding, and then biweekly until the plants entered their reproductive stage. To estimate Fe removed by plants, AMD samples were taken from both planted and unplanted systems for total dissolved Fe analyses. The data revealed some remediation effects of planting aquatic plants on AMD in the wetland treatment systems. The presence of plants in the wetland system appeared to induce oxygen diffusion to surrounding roots, which might result in Fe precipitation on root surface. Although no differences among planting treatments, Fe removals by plants highly correlated (R2=0.92) with the production of plant biomass.

Although the roles of plants in a treatment wetland have been generally considered minor (Batty, 2003), a number of works have shown otherwise. Bose et al., (2008), stated that plants play an important role in wetland geochemistry due to their active/passive transport of elements. Metal accumulation by various wetland plants has been documented by other investigators. A study by Cheng et al., (2002), found that Cyperus alternifolius and Vallarsia removed one-third of Cd, Cu, Pb, Mn, and Zn from a wetland.
Similarly Kamal et al., (2004) (Skousen & Ziemkiewicz, 1996;Barry, 2003). Plants increase residence time of the incoming water and thereby increase metal retention within the wetland (Nyquist & Greger, 2003). Plant-mediated transfer of oxygen to the rhizosphere by leakage from roots increases aerobic degradation of organic matter.
Growing plants are also important source of C for providing a food source for the bacteria that are responsible for sulfate reduction, resulting in the immobilization of metals (Skousen & Ziemkiewicz, 1996;Batty, 2003). The vegetated wetlands have additional site-specific values by providing habitats for wildlife and making wastewater treatment systems aesthetically pleasing (Brink, 1994;Skousen & Ziemkiewicz, 1996;Batty, 2003).
Limited plant species in Indonesia have been studied, particularly for the purpose of acid mine drainage remediation in a wetland system. An initial work by Juhaeti et al., (2005) and relatively high dissolved Fe concentration (Munawar, 2007). Therefore, further study on the use of aquatic plants in a treatment wetland is needed.
This study was intended to investigate the remediation effects of planting three aquatic plants

MATERIALS AND METHODS
This study was conducted in a wire house belongs to the Faculty of Agriculture, University of Bengkulu. A bench-scale wetland cell simulation was constructed using 10 L-plastic containers, organic substrate mixtures of 25% chicken manure + 25% sludge + 50% bark as growth media, three aquatic plants species, and poor AMD. The chicken manure was obtained from local farmers in Bengkulu, while sludge and bark were supplied by a pulp and paper industry (PT Tanjung Enim Lestari), Tanjung Enim, South Sumatra. All organic materials were passed through 5 mm sieve to obtain uniform materials, mixed in their respective portions, and air-dried. The growth media were obtained by weighing 4 kg of the organic mixtures, and then were added with 0.4 kg of soil mud for media stabilization. The collected data were statistically analyzed using Analysis of Variance (ANOVA) test. If the treatments showed significant differences, their mean treatment effects were tested using Least Significant Difference (LSD) test at á = 0.05.

RESULTS AND DISCUSSIONS
Acid mine drainage pH. Overall pH values of the AMD in all planting treatments and the control during the experiment were much higher than its original pH (2.80) as presented in Figure 1a. This means that the substrates in the growth media had played important role in raising the pH, regardless the planting treatments. Such observations have been reported by various studies (Waybrant et al., 1998;Gibert et al., 2004;Zagury et al., 2006;Sheoran, 2006;Munawar & Riwandi, 2010). Alkaline manure and sludge contained in the growth media might have contributed their alkalinity to the system. Besides, the organic media could also have provided sources of C for sulfatereducing bacteria (SRB) to develop, resulting in sulfate reduction to sulfides which then reduced acidity and hence increased alkalinity (pH). This agrees with a study by Sheoran (2006), who found significant pH increases (from around 3 to more than 7 in a wetland system using wood chips and goat manure. In a previous study we also found that organic wastes containing high alkalinity (high pH) and nutrient concentrations (chicken manure) improved AMD quality, through increasing its pH above 6 (Munawar & Riwandi, 2010). Drastic increase from pH 2.8 of the original AMD to more than 6 occurred 24 hours after treatment application. Thereafter, the pH of AMD under all planting treatments was relatively constant at levels between 6 and 7 until the end of the experiment.
Our statistical analysis (P<0.05) found that the significant effects of overall planting treatments occurred at 24 hours (1 st day) and on 56 th day after planting. After 24 hours the pH values of AMD in all the planted media (pH 6.15-6.25) were significantly higher than in the unplanted media (pH 5.97), although there were no significant differences among the planting treatments. Similar results were reported by Sheoran (2006), who found pH increases from about 3 to more than 7 after 24 hours of planting three plant species (Typha angustifolia, Desmostachya bipinnata, and Sacharum bengalense) in media consisting of 75% soil, 20% powdered goat manure, and 5% wood shavings. He attributed these increases to the sulfide precipitation, generating alkalinity in the system.